Microorganisms and Disease
The influenza vaccine is administered each year because

the vaccine is sufficiently toxic to make it necessary to administer only a small amount at any one time
mutations in the viral hemagglutinin may allow the virus to evade the immune response elicited by previous vaccines
None of these
it is a polysaccharide vaccine that does not confer long-term protection

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Microorganisms and Disease
Plastic implants can pose a serious nosocomial infection problem because

phagocytes have trouble moving on the plastic surface
phagocytes have trouble engulfing bacteria embedded in a biofilm
All of these
infected implants usually have to be surgically removed

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